India’s Punjab National Bank (PNB) is conducting an internal investigation into an alleged $2 billion fraud, vowing to come down on anyone involved in the crime.

“We are examining every aspect of it, and if we find others involved, we will not spare them,” said PNB CEO Sunil Mehta, according to Reuters.

In February, PNB announced that it had discovered $1.8 billion in fraudulent activity at one of its branches.

The scandal involves firms associated with diamond merchants Nirav Modi, the billionaire jeweler, and his uncle, Mehul Choksi, who heads jewelry retailer Gitanjali Gems. The allegations, which go as far back as 2010, involve the firms receiving fraudulent letters of undertaking (LoUs) from the Brady House branch, which allowed them to borrow money from overseas banks.

Last month, a source said the scandal could hit the $2 billion mark.

So far, 20 people have been arrested, and a court has issued non-bailable warrants against jewelers Modi and Choksi. They both have denied the allegations against them, and left the country before the fraud was uncovered.

For its part, PNB has suspended 21 officials, and Mehta said the bank was investigating how the fraud managed to go undetected for nearly a decade.

In addition, several experts have questioned how external auditors approved by India’s central bank failed to notice the illegalities. However, these firms often only do top-line reviews.

“I don’t blame them. They come only for a short period of time and they go and basically see data only on the CBS,” said Mehta, referring to PNB’s core banking software.

PNB explained that those responsible hid the fraud by issuing the guarantees via the SWIFT interbank messaging system and not entering the corresponding entries in the CBS. The bank is in the process of integrating its CBS with SWIFT so this cannot happen again.

In addition, PNB will add more checks to prevent a handful of employees from carrying out the same type of fraud.

“Earlier, we were dependent mainly on physical inspection of the branches. Now we’re going to create an offline monitoring cell. Besides the physical inspection, we will also monitor all exceptional transactions,” Mehta said.